Article "the" with geographical names

.... Pacific Ocean has many different types of fish.

I've never been to ..... United States.

.... Mount Everest is the highest mountain in the world.

.... Grampian Mountains are a mountain range of central Scotland.

.... United Kingdom of Great Britain and Northern Ireland is situated off the north-west coast of Europe.

I went sailing around .... Lake Geneva.

The most important rivers are (1).... Thames, (2).... Severn and (3)... Clyde.

The UK is separated from the continent by (1)... English Channel and (2).... Strait of Dover.

Would you like to visit .... South America?

The capital of _______Republic of Ireland is _______Dublin.

Ireland is situated in ______ north-west of _____Europe.

I spent a year travelling around_________ Europe.

Will we use the definite article “the” or zero article before rivers?

Will we use the definite article “the” or zero article before group of mountains?

Will we use the definite article “the” or zero article before a mountaintop?

Will we use the definite article “the” or zero article before deserts?

Will we use the definite article “the” or zero article before continents?

Will we use the definite article “the” or zero article before towns?

Will we use the definite article “the” or zero article before countries whose name includes a word like REPUBLIC OR UNITED?

Will we use the definite article “the” or zero article before group of lakes?

Will we use the definite article “the” or zero article before oceans?

Will we use the definite article “the” or zero article before groups of islands?